This invention relates to planar lightwave circuits. The invention has particular application to silica on silicon type lightwave circuits. Such circuits are fabricated on a silicon substrate attached to a support plate (which is referred to herein as a xe2x80x9criserxe2x80x9d).
A typical planar lightwave circuit (PLC) comprises light guides, optical switches, optical gratings or other optical components formed in an optical layer on a substrate. The substrate typically comprises a silicon wafer. In silica on silicon technology, one or more layers of doped or undoped silica are deposited on a silicon substrate. The optical components are fashioned in the silica layers. The optical layer typically comprises at least three or more layers of silica including a bottom cladding layer, a core layer and a top cladding layer. Some types of PLC include silicon in the optical layer, as is the case with silicon-on-insulator (SOI) PLCs. Other alternative PLC constructions have optical layers based upon other materials. These include InP-based PLCs, GaAs-based PLCs and polymer based PLCs.
Not all PLCs that are manufactured are xe2x80x9cgoodxe2x80x9d in that they meet specified performance criteria. Typically a number of PLCs are fabricated on a silicon wafer. The as-manufactured PLCs are tested. PLCs which meet the specified performance criteria are selected for packaging.
PLCs are typically packaged in assemblies which include a riser on which the substrate is mounted. The riser provides a convenient base for mounting the PLC, mechanical support for the PLC and a way to maintain the PLC at a desired temperature. The riser is typically made from a thermally conductive material. Thermally conductive materials suitable for use in making risers include various metals. A typical metal suitable for making risers is CuW. A heater and temperature sensor may be attached to the riser for the purpose of maintaining the PLC at a desired operating temperature. The substrate is affixed to the riser with an adhesive, typically a thermally cured epoxy.
Differences in the coefficient of thermal expansion of the substrate and the riser can cause the PLC to become distorted. Typically the distortion is associated with a difference between the temperature at which the epoxy was cured and the temperature of the PLC during its operation. The distortion causes stresses in the optical components. Such stresses may alter the optical properties of the optical components. For example, a change in stress in the material which makes up an arrayed waveguide grating may alter the effective index and birefringence of the material. This can alter performance characteristics of the PLC including the center wavelength of the passbands of the arrayed waveguide grating. Since such performance characteristics may need to meet performance specifications for the PLC, variations in such performance characteristics from design values are highly undesirable. The degradation of the performance characteristics of the PLC is a direct result of the distortion. The stress-induced variations in performance can cause a PLC which met its performance specifications prior to packaging to fail after it has been packaged.
The extent to which a PLC will be distorted depends upon a large number of factors including the temperature at which the adhesive used to attach the substrate to the riser is cured and the thickness, stiffness and coefficient of thermal expansion of each relevant layer. These layers include the substrate, the riser, the adhesive and, to a smaller degree, the optical layer. The values of each of these parameters will vary somewhat between different PLCs due to manufacturing variations. As a result of such manufacturing variations, the effect of stress on the operational characteristics of a PLC are difficult to predict to the desired degree of accuracy
So far, there have been attempts to address this problem by using low modulus adhesives to join the substrate and riser. While such adhesives do reduce the amount of distortion and, may consequently reduce stress-induced changes in the performance characteristics of PLCs, they do not eliminate the problem. Furthermore, such low modulus adhesives may have other undesirable characteristics.
There remains a need for a way to avoid the degradation in performance caused by thermally induced distortions of PLCs.
This invention provides lightwave circuit assemblies comprising a riser, a substrate on a first face of the riser and a lightwave circuit on the substrate. Lightwave circuit assemblies according to the invention have a layer of material on a second face of the riser. The layer of material counters thermally induced distortions of the riser.
Accordingly, a first aspect of the invention provides a planar lightwave circuit assembly comprising: a riser, a substrate affixed to a first face of the riser, an optical layer comprising a lightwave circuit on the substrate and, a layer of material affixed to a second face of the riser. The second face is opposed to the first face. The riser has a first coefficient of thermal expansion, The substrate has a second coefficient of thermal expansion different from the first coefficient of thermal expansion and the layer of material has a third coefficient of thermal expansion different from the first coefficient of thermal expansion. The second and third coefficients of expansion may be the same. The second and third coefficients of thermal expansion are either both greater than the first coefficient of thermal expansion or both less than the first coefficient of thermal expansion. In preferred embodiments of the invention the layer of material has a coefficient of thermal expansion substantially the same as the coefficient of thermal expansion of the substrate. Most preferably the layer of material and the substrate each comprise the same material, and the layer of material and the substrate have substantially equal thicknesses. The layer of material and the substrate may advantageously each comprise a silicon layer.
Preferably the substrate and layer of material are each attached to the riser with a layer of a thermally activated epoxy. The layers of thermally activated epoxy attaching the substrate and layer of material to the riser are preferably substantially equal in thickness.
The planar lightwave circuit assembly may comprise a heater and/or temperature sensor in thermal connection with the riser. Preferably one of the faces of the riser has a first cut away portion to accommodate a heater or temperature sensor and the other one of the faces has a corresponding second cut away portion. There may be multiple pairs of corresponding cut away portions on the faces of the riser.
Another aspect of the invention provides a planar lightwave circuit assembly comprising: a riser; a substrate having a coefficient of thermal expansion; an optical layer comprising a lightwave circuit on the substrate; first adhesive means for affixing the substrate to the riser; a layer of material having a coefficient of thermal expansion substantially the same as the coefficient of thermal expansion of the substrate; and, second adhesive means for affixing the layer of material to the riser. Preferably the substrate covers a first region on a first face of the riser and the layer of material covers a second region substantially coextensive with the first region on a second face of the riser.
The planar lightwave circuit assembly preferably comprises thermal regulation means for maintaining the planar lightwave circuit assembly at an operating temperature. The optical layer may comprise a plurality of layers of silica and the substrate may comprise a silicon layer. In currently preferred embodiments of the invention, the layer of material and the substrate each comprise a silicon layer and, the layer of material and the substrate have substantially equal thicknesses.
The first and second adhesive means preferably respectively comprise first and second layers of a thermally activated epoxy which are substantially equal in thickness.
A planar lightwave circuit assembly according to the invention preferably includes mounting means for mounting the planar lightwave circuit to a submount. The mounting means may comprise mounting portions of the riser, the mounting portions projecting outwardly past edges of the substrate and layer of material.
Further features and advantages of the invention are described in the following detailed description of the invention and the drawings.